This invention relates to a transmission for a tractor.
A known power transmission system of a tractor includes a change speed device that changes the speed of power from an engine for transmission to drive wheels, a reverse and forward drive switching device (sometimes referred to as shuttle shift) disposed between the engine and change speed device for switching a rotating direction of the power. The reverse and forward drive switching device can change forward drive to two, high and low, speeds, and includes a high-speed forward drive clutch for switching the power from the engine to a forward high speed, a low-speed forward drive clutch for switching the power to a forward low speed, and a reverse drive clutch for switching the power to reverse drive. These clutches are in the form of wet type multi-disk hydraulic clutches (see Publication JP4-107348 of Japanese Patent Application).
In this power transmission system, the high-speed forward drive clutch is mounted on a main drive shaft directly coupled to an engine output shaft. The low-speed forward drive clutch and reverse drive clutch are mounted on a counter shaft disposed below the main drive shaft.
Each wet type multi-disk hydraulic clutch is constructed to transmit power by supplying pressure oil to the back of a piston to move the piston and press drive plates and clutch disks against a pressure plate. An oil passage for feeding the pressure oil to the back of the piston is formed in the shaft on which clutch is mounted.
In the conventional power transmission system noted above, the high-speed forward drive clutch is mounted on the main drive shaft, and the low-speed forward drive clutch and reverse drive clutch on the counter shaft. The oil passages for supplying pressure oil must be formed in the respective shafts. Thus, there is room for improvement in design.